Electroless gold plating solution

ABSTRACT

A cyanide-free immersion type electroless gold plating solution that is less toxic can be used at near neutrality and gives an excellent and improved solder adhesion and plated film adhesion is provided. 
     The electroless gold plating solution contains a cyanide-free water-soluble gold compound, a pyrosulfurous acid compound and a thiosulfuric acid compound. This plating solution preferably further contains a sulfurous acid compound and an aminocarboxylic acid compound. Pyrosulfurous acid and alkali metal salts, alkaline-earth metal salts, ammonium salts and other salts thereof can be used as the pyrosulfurous acid compound.

TECHNICAL FIELD

This invention relates to plating technology, and more particularlyrelates to a cyanide-free immersion type electroless gold platingsolution.

BACKGROUND ART

Immersion type electroless gold plating solutions are used forintermediate layers for the purpose of improving the solder adhesion ofcircuits, terminals, and so forth of printed wiring boards and improvingthe adhesion of reductive gold plating and the like. Most of the goldplating solutions employed for this purpose involve a toxic cyanidecompound as the gold compound, but for environmental and workplaceconcerns, a cyanide-free gold plating solution that does not employtoxic substances have been required.

Patent applications that have been filed for cyanide-free immersion typeelectroless gold plating solutions include those that make use of goldsulfite compounds (see, for example, Patent Document 1 and PatentDocument 2), those that make use of a salt of gold sulfites orchloroaurates (see, for example, Patent Document 3), and those that makeuse of gold sulfite, gold chloride, gold thiosulfate, or goldmercaptocarboxylates (see, for example, Patent Document 4). Theelectroless gold plating solutions described therein are cyanide freeand therefore less toxic, and can be used at near neutrality, but theystill have the problems of inferior solder adhesion and plating filmadhesion. “Plating film adhesion” refers to the adhesion between aimmersion type electroless gold plating film and the substrate and, whena immersion type electroless gold plating film is used as anintermediate layer, that refers to the adhesion between the layers aboveand below the film.

-   Patent Document 1: Japanese Patent No. 3,030,113-   Patent Document 2: Japanese Patent Publication No. 2003-13249-   Patent Document 3: Japanese Patent Publication No. H8-291389-   Patent Document 4: Japanese Patent Publication No. H10-317157

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

In view of the foregoing situation, it is an object of this invention toprovide a cyanide-free immersion type electroless gold plating solutionthat exhibits low toxicity, good solder adhesion and good plating filmadhesion and that can be used at around neutrality.

MEANS FOR SOLVING THE PROBLEMS

As a result of investigation for the factors that impairs the solderadhesion and plating film adhesion of immersion type electroless goldplating film, the inventors revealed that the problem is non-uniformsubstitution from the underlying metal film, such as an underlyingnickel film. More specifically, when non-uniform corrosion marks such aspitting were seen in the underlying nickel film after the gold platingfilm had been stripped off, the solder adhesion and plating filmadhesion were poor because defects of some kind were also present in theimmersion type electroless gold plating film. Conversely, the solderadhesion and plating film adhesion were good when non-uniform corrosionmarks were not present.

As a result of investigations into a bath composition that wouldeliminate the non-uniform corrosion marks in the underlying nickel filmafter the gold is stripped off, the inventors have already discoveredthat the addition of a pyrosulfurous acid compound to a cyanide-freewater-soluble gold compound is effective as a cyanide-free immersiontype electroless gold plating solution, thereby, that liquid affordsgold plating films that exhibit an excellent solder adhesion and platingfilm adhesion (see PCT/JP2004/001784). This plating solution improvessolder adhesion and plating film adhesion in comparison to conventionalcyanide-free immersion type electroless gold plating solutions and alsoprovides an excellent adhesive strength with lead-free solder and thelike. However, as a result of further investigations into cyanide-freeimmersion type electroless gold plating that would have an even bettersolder adhesion and plating film adhesion, the inventors found out that,by the addition of a thiosulfuric acid compound along with thepyrosulfurous acid compound as additives for the plating solution, thesolder adhesive strength is additionally enhanced and the reliability isimproved. This invention was achieved based on this discovery.

More specifically, the present invention is as follows.

(1) An immersion type electroless gold plating solution, comprising acyanide-free water-soluble gold compound, a pyrosulfurous acid compoundand a thiosulfuric acid compound.

(2) An immersion type electroless gold plating solution according to(1), further containing a sulfurous acid compound.

(3) An immersion type electroless gold plating solution according to (1)or (2), further containing an aminocarboxylic acid compound.

(4) A gold plated article, produced by the use of an immersion typeelectroless gold plating solution according to any of (1) to (3).

The cyanide-free water-soluble gold compound used in the platingsolution according to the present invention is not particularlyrestricted as long as it is free from cyanide and water soluble, whilethe plating solution is characterized by containing a pyrosulfurous acidcompound and a thiosulfuric acid compound as additives.

EFFECT OF THE INVENTION

The present invention provides a cyanide-free immersion type electrolessgold plating solution that has a low toxicity, that can be used at nearneutrality, and that exhibits an even better solder adhesion and platedfilm adhesion. The present invention in particular provides acyanide-free immersion type electroless gold plating solution thatimproves the adhesive strength with lead-free solder, which showed lowadhesive strength.

BEST MODE FOR CARRYING OUT THE INVENTION

The immersion type electroless gold plating solution according to thepresent invention is explained in detail hereinbelow.

The electroless gold plating solution according to the present inventionis prepared by dissolving a cyanide-free water-soluble gold compound, apyrosulfurous acid compound, and a thiosulfuric acid compound in water.

The cyanide-free water-soluble gold compound should be a cyanide-freegold compound but is not otherwise particularly restricted. As thecompound, gold sulfite, gold thiosulfate, gold thiocyanate, chloroauricacid, or a salt of the preceding is preferably used. Usable as the saltare alkali metal salts, alkaline-earth metal salts, ammonium salt and soforth, and sodium salt, potassium salt, ammonium salt and so forth arepreferred. The electroless gold plating solution according to thepresent invention contains these gold compounds preferably at 0.1 to 100g/L and more preferably 0.5 to 20 g/L as the gold concentration in theplating solution. The gold substitution rate slows substantially at agold concentration less than 0.1 g/L, while values above 100 g/L aremeritless with no further improvement.

Usable as the pyrosulfurous acid compound are pyrosulfurous acid and itsalkali metal salts, alkaline-earth metal salts, ammonium salt and soforth, and preferably sodium pyrosulfite, potassium pyrosulfite,ammonium pyrosulfite and so forth are used. The plating solutioncontains preferably 0.1 to 200 g/L of the pyrosulfurous acid compoundand more preferably 1 to 100 g/L. The effect to prevent non-uniformcorrosion of the underlying nickel is weak when the pyrosulfurous acidcompound concentration is less than 0.1 g/L, while values in excess of200 g/L brings no further advantage.

Usable as the thiosulfuric compound are the alkali metal salts,alkaline-earth metal salts, ammonium salt, and so forth of thiosulfuricacid, and preferably sodium thiosulfate, potassium thiosulfate, ammoniumthiosulfate and so forth are employed. The plating solution includespreferably 1 mg/L to 10 g/L of the thiosulfuric acid compound and morepreferably 10 to 1000 mg/L. There is little improvement in the solderadhesive strength when the thiosulfuric acid compound concentration isless than 1 mg/L, while values above 10 g/L brings no further merit.

The electroless gold plating solution according to the present inventionalso preferably includes a sulfurous acid compound as a stabilizer. Asthe sulfurous acid compound, sulfurous acid and its alkali metal salts,alkaline-earth metal salts, ammonium salt and so forth are exemplified.The concentration of the sulfurous acid compound in the plating solutionis preferably 0.1 to 200 g/L and more preferably is 1 to 100 g/L. Theactivity as a stabilizer does not appear at less than 0.1 g/L, whilevalues over 200 g/L are meritless with no further improvement.

The gold plating solution of the present invention may also contain anaminocarboxylic acid compound as a complexing agent. Examples of theaminocarboxylic acid compound includes ethylenediaminetetraacetic acid,hydroxyethylethylenediaminetriacetic acid,dihydroxyethylethylenediaminediacetic acid, propanediaminetetraaceticacid, diethylenetriaminepentaacetic acid, triethylenetetraminehexaaceticacid, glycine, glycylglycine, glycylglycylglycine,dihydroxyethylglycine, iminodiacetic acid, hydroxyethyliminodiaceticacid, nitrilotriacetic acid, and nitrilotripropionic acid, and thealkali metal salts, alkaline-earth metal salts, ammonium salts and soforth of the preceding. The concentration of the aminocarboxylic acidcompound in the plating solution is preferably 0.1 to 200 g/L and morepreferably 1 to 100 g/L. The effect as a complexing agent is poor whenthe concentration of the aminocarboxylic acid compound is less than 0.1g/L, while values in excess of 200 g/L provides no further advantage.

A phosphoric acid compound may also be added as a pH buffer to theelectroless gold plating solution according to the present invention, asnecessary.

As the phosphoric acid compound, phosphoric acid and pyrophosphoric acidand alkali metal salts alkaline-earth metal salts and ammonium saltsthereof, and alkali metal dihydrogenphosphates, alkaline-earth metaldihydrogenphosphates, ammonium dihydrogenphosphate, dialkali metalhydrogenphosphates, alkaline-earth metal hydrogenphosphates, diammoniumhydrogenphosphate and so forth are exemplified. The concentration of thephosphoric acid compound in the plating solution is preferably 0.1 to200 g/L and more preferably 1 to 100 g/L.

The pH of the gold plating solution according to the present inventionis adjusted using the aforementioned compounds as pH buffers preferablyto pH 4 to 10 and more preferably to pH 5 to 9.

The gold plating solution of the present invention is preferably used ata bath temperature of 10 to 95° C. and more preferably 50 to 85° C.

Problems such as a slow plating rate and facile bath decomposition canoccur when the pH and bath temperature of the plating solution areoutside of the ranges given above.

A plating film which was plated by use of the gold plating solution ofthe present invention after an underlying nickel plating has been platedon a printed wiring board exhibits an excellent solder adhesion andplated film adhesion due to the absence of non-uniform substitution fromthe underlying nickel plating film. Non-uniform corrosion marks are notseen in the underlying nickel film after the gold plating film isstripped off.

EXAMPLES

Preferred embodiments of this invention are described by the examplesand comparative examples provided below.

Examples 1 and 2

Plating solutions with the compositions given in Table 1 were preparedas the immersion type electroless gold plating solution. A copper-cladprinted wiring board with 0.4 mm-diameter resist openings was used as amaterial to be plated and it was plated by the following process.

-   -   acid degreasing (45° C., 5 minutes)    -   → soft etching (25° C., 2 minutes)    -   → acid rinse (25° C., 1 minute)    -   → activator (KG-522 made by Nikko Metal Plating) (25° C.,        pH<1.0, 5 minutes)    -   → acid rinse (25° C., 1 minute)    -   → electroless nickel-phosphorus plating        -   (plating solution: KG-530 made by Nikko Metal Plating,        -   grade: the phosphorous concentration in the plated film was            approximately 7%)        -   (88° C., pH 4.5, 30 minutes)    -   → immersion type electroless gold plating (plating solution and        plating conditions given in Table 1)    -   → reductive electroless gold plating        -   (plating solution: KG-560 made by Nikko Metal Plating)        -   (70° C., pH 5.0, 30 minutes)    -   (a one-minute water rinse step is inserted between all steps        except acid rinse → activator)

The plated articles thus obtained were evaluated as follows.

For the state of corrosion of the underlying nickel plating film, thepresence/absence of pitting corrosion marks was visually checked by SEMat 2000× magnification after the immersion type electroless gold platingfilm had been stripped off using Aurum Stripper 710 (25° C., 0.5minute), a gold stripper made by Nikko Metal Plating.

For the solder adhesive strength, a 0.4 mm-diameter Sn-37Pb solder ballwas mounted after the immersion type electroless gold plating procedurehad been carried out, and the ball was adhered by heating at a peaktemperature of 240° C. in a reflow oven. The solder adhesive strengthwas then measured by the hot bump pull method using a series 4000 bondtester made by Dage.

For the plated film adhesion, reductive electroless gold plating wasperformed after the immersion type electroless gold plating, and thepresence/absence of plated film peeling was then visually scored after atape peel testing. The peel test is a test in which cellophane tape(Cellotape® made by Nichiban) is adhered to the plated film; the tape isthen peeled off; and the presence/absence of adhesion by the plated filmto the tape is visually checked.

The thickness of the plated film was measured using an SFT-3200fluorescent x-ray coating thickness gauge made by Seiko Denshi Kogyo.

The results of the evaluations are given in Table 1.

Examples 3 and 4 and Comparative Examples 1 and 2

Plating solutions with the compositions given in Table 1 were preparedas the immersion type electroless gold plating solution, and platedarticles were fabricated by the same plating process as Example 1.

The solder adhesive strength was measured in the same way as Example 1,but in this case using a 0.4 mm-diameter Sn-3.0Ag-0.5Cu lead-free solderball and bonding by heating at a peak temperature of 250° C. in thereflow oven. The results of the evaluations are given in Table 1.

Examples 5 and 6

Plated articles were fabricated by the same plating procedure as inExample 1, except for the conditions of the electrolessnickel-phosphorus plating and the immersion type electroless goldplating, i.e., the latter immersion type electroless gold platingcondition was given in Table 1 and the former was the following:

-   -   plating solution: KG-571 made by Nikko Metal Plating,    -   grade: the phosphorous concentration in the plated film is        approximately 9%; and    -   plating conditions: 80° C., pH 4.6, 30 minutes.

The solder adhesive strength was measured in the same manner as Example1, but in this case using a 0.4 mm-diameter Sn-3.0Ag-0.5Cu lead-freesolder ball and adhering by heat at a peak temperature of 250° C. in thereflow oven. The results of the evaluations are given in Table 1.

Examples 7 to 10

Plated articles were fabricated by plating in the same procedure asExample 3 except for the plating solutions with the compositions givenin Table 1, which were used instead of the immersion type electrolessgold plating solution used in Example 3. The articles were alsoevaluated in the same way as Example 3, and the results of theevaluations are given in Table 1.

TABLE 1 Examples 1 2 3 4 Bath Gold Sodium gold sulfite: Sodium Sodiumgold sulfite: Sodium composition compound 1 g/L (gold) chloroaurate: 1g/L (gold) chloroaurate: 1 g/L (gold) 1 g/L (gold) Additive Sodiumpyrosulfite: Sodium pyrosulfite: Sodium pyrosulfite: Sodium pyrosulfite:5 g/L 10 g/L 5 g/L 10 g/L Additive Sodium thiosulfate: Sodiumthiosulfate: Sodium thiosulfate: Sodium thiosulfate: 50 mg/L 100 mg/L 50mg/L 100 mg/L Stabilizer Sodium sulfite: Sodium sulfite: Sodium sulfite:5 g/L Sodium sulfite: 5 g/L 10 g/L 10 g/L Complexing Ethylenediamine-Nitrilotriacetic Ethylenediamine- Nitrilotriacetic agent tetraaceticacid: acid: 10 g/L tetraacetic acid: acid: 10 g/L 10 g/L 10 g/L pHbuffer Sodium dihydrogen- Disodium hydrogen- Sodium dihydrogen- Disodiumhydrogen- phosphate: 30 g/L phosphate: 30 g/L phosphate: 30 g/Lphosphate: 30 g/L Plating pH 7.5 7.5 7.5 7.5 conditions Treatment 80 8080 80 temp. (° C.) Treatment 20 20 20 20 time (min) Evaluation Film 0.050.05 0.05 0.05 results thickness (μm) Pitting no no no no Solder adh.1641 1610 1423 1402 strength Plated film no peeling no peeling nopeeling no peeling adhesion Comparative Examples Examples 1 2 5 6 BathGold Sodium gold sulfite: Sodium Sodium gold sulfite: Sodium compositioncompound 1 g/L (gold) chloroaurate: 1 g/L (gold) chloroaurate: 1 g/L(gold) 1 g/L (gold) Additive Sodium pyrosulfite: Sodium pyrosulfite:Sodium pyrosulfite: Sodium pyrosulfite: 5 g/L 10 g/L 5 g/L 10 g/LAdditive — — Sodium thiosulfate: Sodium thiosulfate: 50 mg/L 100 mg/LStabilizer Sodium sulfite: Sodium sulfite: Sodium sulfite: Sodiumsulfite: 5 g/L 10 g/L 5 g/L 10 g/L Complexing Ethylenediamine-Nitrilotriacetic Ethylenediamine- Nitrilotriacetic agent tetraaceticacid: acid: 10 g/L tetraacetic acid: acid: 10 g/L 10 g/L 10 g/L pHbuffer Sodium dihydrogen- disodium hydrogen- Sodium dihydrogen- disodiumhydrogen- phosphate: 30 g/L phosphate: 30 g/L phosphate: 30 g/Lphosphate: 30 g/L Plating pH 7.5 7.5 7.5 7.5 conditions Treatment 80 8080 80 temp. (° C.) Treatment 20 20 20 20 time (min) Evaluation Film 0.050.05 0.05 0.05 results thickness (μm) Pitting no no no no Solder adh.1216 1195 1587 1536 strength Plated film no peeling no peeling nopeeling no peeling adhesion Examples 7 8 9 10 Bath Gold Sodium goldsulfite: Sodium Sodium gold sulfite: Sodium composition compound 1 g/L(gold) chloroaurate: 1 g/L (gold) chloroaurate: 1 g/L (gold) 1 g/L(gold) Additive Sodium pyrosulfite: Sodium pyrosulfite: Sodiumpyrosulfite: Sodium pyrosulfite: 1 g/L 2 g/L 50 g/L 100 g/L AdditiveSodium thiosulfate: Sodium thiosulfate: Sodium thiosulfate: Sodiumthiosulfate: 10 mg/L 100 mg/L 50 mg/L 75 mg/L Stabilizer Sodium sulfite:Sodium sulfite: Sodium sulfite: Sodium sulfite: 10 g/L 15 g/L 10 g/L 15g/L Complexing Ethylenediamine- Nitrilotriacetic Ethylenediamine-Nitrilotriacetic agent tetraacetic acid: acid: 10 g/L tetraacetic acid:acid: 5 g/L 10 g/L 20 g/L pH buffer Trisodium Disodium hydrogen- Sodiumdihydrogen- Potassium phosphate: phosphate: phosphate: pyrophosphate: 30g/L 20 g/L 30 g/L 40 g/L Plating pH 7.5 7.5 7.5 7.5 conditions Treatment80 80 80 80 temp. (° C.) Treatment 20 20 20 20 time (min) EvaluationFilm 0.05 0.05 0.05 0.05 results thickness (μm) Pitting no no no noSolder adh. 1421 1438 1467 1455 strength Plated film no peeling nopeeling no peeling no peeling adhesion Unit for the solder adhesivestrength: gf (n = 20)

The results in Table 1 demonstrate that the gold plating solutions usedin the comparative examples, which are electroless gold platingsolutions described in PCT/JP2004/001784, provide plated articles thatare free of pitting and that therefore exhibit an excellent solderadhesion and plated film adhesion. However, the results in Table 1 alsodemonstrate that the electroless gold plating solutions of the presentinvention bring solder adhesion much more improved than the gold platingsolutions in the comparative examples.

1. A substitutional electroless gold plating solution, consistingessentially of a cyanide-free water-soluble gold compound, apyrosulfurous acid compound and a thiosulfuric acid compound selectedfrom the group consisting of an alkali metal salt of thiosulfuric acid,an alkaline earth metal salt of thiosulfuric acid and an ammonium saltof thiosulfuric acid, wherein the concentration of the thiosulfuric acidcompound is from 1-1000 mg/l.
 2. A substitutional electroless goldplating solution according to claim 1, further containing a sulfurousacid compound.
 3. A substitutional electroless gold plating solutionaccording to claim 1, further containing an aminocarboxylic acidcompound.